1. Field of the Invention
The invention relates to lithographic processes, and in particular, to lithographic processes involving device fabrication.
2. Art Background
Lithographic processes are typically employed in the manufacture of devices such as semiconductor devices. Since the exposure occurs simultaneously over an entire device or a number of devices being processed on a substrate, e.g., a silicon substrate, the procedure is considered a blanket exposure. That is, a material, i.e. a resist, that is sensitive to the exposing light is coated onto a substrate, e.g., a silicon wafter, that is being processed to form a plurality of devices. The coating material is (if desired) baked, subjected to spatially discrete radiation, e.g., light that has been passed through a mask material so that the light reaching the resist corresponds to a desired pattern that is to be transferred into the underlying substrate, and then post-exposure baked before development of the pattern.
Resist materials, including a polymer having a protective group, have been described in U.S. Pat. No. 4,812,542, dated Mar. 14, 1989. The protective group present in these materials is employed in the synthesis process to prevent undesired reactions during formation of the resist material. Such protective groups, since they are not involved in the operation of the resist, are removed before or after exposure throughout the resist material by an expedient such as heating. Thus, for example, in a synthesized polymer, such as poly(4-tert-butoxycarbonyloxystyrene-co-o-nitro-.alpha.-methylbenzyl methacrylate), the tert-butoxycarbonyl protective group is removed to leave a hydroxyl substituent. This substituent then performs the function of providing an acid moiety that after exposure and bake provides alkaline solubility to the exposed regions. The protective group, however, is not involved in producing the desired pattern.
Among the lithographic processes that are available to expose resists having protective groups, photolithography is often utilized. Photolithographic processes have the advantage of being suitable for a blanket exposure technique. A blanket exposure is advantageous because it is relatively fast compared to other methods such as the raster scan technique usually employed when the energy used to expose the resist is a beam of electrons. However, generally, resolution obtainable through a blanket exposure with near ultraviolet or visible light is somewhat poorer than that achieved with other methods such as electron lithography.
Improved resolution with a blanket exposure is achievable by using deep ultraviolet or X-ray light. X-ray exposure generally has a potential for better resolution than exposure with deep ultraviolet light, but has not been studied as extensively. One approach to a photoresist sensitive to deep ultraviolet radiation employs a compound that produces an acid moiety upon irradiation together with a polymer that reacts under the influence of heat with the generated acid. This reaction is often through a protective group that is involved in the lithographic process and that is removed from the polymer to form an acidic moiety such as a hydroxyl or carboxylic acid group.
Typical acid generator/acid sensitive polymer combinations include an onium salt as the photosensitive acid generator and a polymer such as poly(4-t-butoxycarbonyloxystyrene) that has a reactive substituent, e.g., a t-butoxycarbonyl protective group. (See Ito, et al. U.S. Pat. No. 4,491,628 dated Jan. 1, 1985.) Such systems are generally referred to as chemical amplification systems since the production of one molecule of acid by actinic radiation induces a reaction in a plurality of molecules in the acid sensitive polymer.
Attempts have been made to improve the sensitivity of chemically amplified resists. Protective groups have been used in materials such as described in Canadian Patent Application 2,001,384. In this patent application, a polymer containing protective groups is solvated and the solution treated by addition of acid with heating to remove a portion of the protective groups. Acid is removed from the resulting partially deprotected polymer, the polymer is then isolated, a photoacid generator is added with a spinning solvent to the polymer, and then this combination is employed to coat the substrate which is subsequently exposed. This procedure is not desirable because it requires an additional reaction after synthesis and a meticulous removal of acid before coating.
To enhance the sensitivity of acid generator/polymer combination, another proposal employs a polymer including both a substituent sensitive to acid and a moiety present in the polymer chain that induces upon irradiation chain scission with associated decrease in molecular weight. As described by R. G. Tarascon, et al., Proceedings of Regional Technical Conference on Photopolymers, Principles, Processes and Materials, Mid Hudson Section, Society of Plastic Engineers, Oct. 30 to Nov. 2, 1988, Ellenville, N.Y., page 11 and R. G. Tarascon et al., Polymer Engineering and Science, 29, 850(1989), one such combination includes an acid generator and a polymer having a sulfone moiety in the backbone.
Although chemically amplified resists, such as those involving poly(4-t-butoxycarbonyloxystyrene), show great promise for fine line exposure, these materials have demonstrated a tendency to shrink upon exposure and post-exposure baking. Such shrinking produces a loss of image quality and, in part, counteracts resolution improvement available through use of ultraviolet, X-ray, or electron beam exposure. Thus, although chemically amplified resists are extremely promising, some improvement is desirable.